A reactive distillation process for producing a dialkyl carbonate and a diol through reaction between a cyclic carbonate and an aliphatic monohydric alcohol was first disclosed by the present inventors (see Patent Document 1: Japanese Patent Application Laid-Open No. H4-198141. Patent Document 2: Japanese Patent Application Laid-Open No. H4-230243. Patent Document 3: Japanese Patent Application Laid-Open No. H9-176061. Patent Document 4: Japanese Patent Application Laid-Open No. H9-183744 Patent Document 5: Japanese Patent Application Laid-Open No. H9-194435. Patent Document 6: International Publication No. WO97/23445 (corresponding to European Patent No. 0889025, U.S. Pat. No. 5,847,189). Patent Document 7: International Publication No. WO99/64382 (corresponding to European Patent No. 1086940, U.S. Pat. No. 6,346,638). Patent Document 8: International Publication No. WO00/51954 (corresponding to European Patent No. 1174406, U.S. Pat. No. 6,479,689). Patent Document 9: Japanese Patent Application Laid-Open No. 2002-308804 and Patent Document 10: Japanese Patent Application Laid-Open No. 2004-131394), patent applications in which such a reactive distillation system is used have subsequently also been filed by other companies (see Patent Document 11: Japanese Patent Application Laid-Open No. H5-213830 (corresponding to European Patent No. 0530615, U.S. Pat. No. 5,231,212). Patent Document 12: Japanese Patent Application Laid-Open No. H6-9507 (corresponding to European Patent No. 0569812, U.S. Pat. No. 5,359,118). Patent Document 13: Japanese Patent Application Laid-Open No. 2003-119168 (International Publication No. WO03/006418). Patent Document 14: Japanese Patent Application Laid-Open No. 2003-300936 and Patent Document 15: Japanese Patent Application Laid-Open No. 2003-342209). In the case of using a reactive distillation system for this reaction, the reaction can be made to proceed with a high conversion. However, reactive distillation processes proposed hitherto have related to producing the dialkyl carbonate and the diol either in small amounts or for a short period of time, and have not related to carrying out the production on an industrial scale stably for a prolonged period of time. That is, these processes have not attained the object of producing a dialkyl carbonate continuously in a large amount (e.g. not less than 2 ton/hr) stably for a prolonged period of time (e.g. not less than 1000 hours, preferably not less than 3000 hours, more preferably not less than 5000 hours).
For example, the maximum values of the height (H: cm), diameter (D: cm), and number of stages (n) of the reactive distillation column, the produced amount P (kg/hr) of dimethyl carbonate, and the continuous production time T (hr) in examples disclosed for the production of dimethyl carbonate (DMC) and ethylene glycol (EG) from ethylene carbonate and methanol are as in Table 1.
TABLE 1PATENTDOCUMENTH:cmD:cmNO. STAGES:nP:kg/hrT:hr11002300.10640041605400.427NOTE 551605400.473NOTE 572004PACKING COLUMN (Dixon)0.932NOTE 58NOTE 15600.275NOTE 59NOTE 15600.258NOTE 510NOTE 15600.258NOTE 5112503PACKING COLUMN (Raschig)0.392NOTE 512NOTE 2NOTE 2NOTE 20.532NOTE 513NOTE 3NOTE 342NOTE 4NOTE 514NOTE 3NOTE 3303750NOTE 51520015PACKING COLUMN (BX)0.313NOTE 5NOTE 1: OLDERSHAW DISTILLATION COLUMN.NOTE 2: NO DESCRIPTION WHATSOEVER DEFINING DISTILLATION COLUMN.NOTE 3: ONLY DESCRIPTION DEFINING DISTILLATION COLUMN IS NUMBER OF STAGES.NOTE 4: NO DESCRIPTION WHATSOEVER OF PRODUCED AMOUNT.NOTE 5: NO DESCRIPTION WHATSOEVER REGARDING STABLE PRODUCTION FOR PROLONGED PERIOD OF TIME.
In Patent Document 14 (Japanese Patent Application Laid-Open No. 2003-300936), it is stated at paragraph 0060 “The present example uses the same process flow as for the preferred mode shown in FIG. 1 described above, and was carried out with the object of operating a commercial scale apparatus for producing dimethyl carbonate and ethylene glycol through transesterification by a catalytic conversion reaction between ethylene carbonate and methanol. Note that the following numerical values in the present example can be adequately used in the operation of an actual apparatus”, and as that example it is stated that 3750 kg/hr of dimethyl carbonate was specifically produced. The scale described in that example corresponds to an annual production of 30,000 tons or more, and hence this implies that at the time of the filing of the patent application for Patent Document 14 (Japanese Patent Application Laid-Open No. 2003-300936) (Apr. 9, 2002), operation of the world's first large scale commercial plant using this process had been carried out. However, even at the time of filing the present application, there is not the above fact at all. Moreover, in the example of Patent Document 14 (Japanese Patent Application Laid-Open No. 2003-300936), exactly the same value as the theoretically calculated value is stated for the amount of dimethyl carbonate produced, but the yield for ethylene glycol is approximately 85.6%, and the selectivity is approximately 88.4%, and hence it cannot really be said that a high yield and high selectivity have been attained. In particular, the low selectivity indicates that this process has a fatal drawback as an industrial production process. (Note also that Patent Document 14 (Japanese Patent Application Laid-Open No. 2003-300936) was deemed to have been withdrawn on Jul. 26, 2005 due to examination not having been requested).
With the reactive distillation method, there are very many causes of fluctuation such as composition variation due to reaction and composition variation due to distillation in the distillation column, and temperature variation and pressure variation in the column, and hence continuing stable operation for a prolonged period of time is accompanied by many difficulties, and in particular these difficulties are further increased in the case of handling large amounts. To continue mass production of a dialkyl carbonate and a diol using the reactive distillation method stably for a prolonged period of time while maintaining high yields and high selectivities for the dialkyl carbonate and the diol, the reactive distillation apparatus must be cleverly devised. However, the only description of continuous stable production for a prolonged period of time with the reactive distillation method proposed hitherto has been the 200 to 400 hours in Patent Document 1 (Japanese Patent Application Laid-Open No. H4-198141) and Patent Document 2 (Japanese Patent Application Laid-Open No. H4-230243).
The present inventors have now established an industrial reactive distillation process that enables a dialkyl carbonate and a diol to be mass-produced continuously and stably for a prolonged period of time with high yield and high selectivity, but to achieve this it has also been necessary to establish an industrial process for separating out the desired dialkyl carbonate from a low boiling point reaction mixture continuously withdrawn in a large amount from an upper portion of the reactive distillation column. The present invention has been devised to attain this object.
As shown in Table 1, with the exception of Patent Document 14 (Japanese Patent Application Laid-Open No. 2003-300936), the produced amount of the dialkyl carbonate per hour using reactive distillation processes proposed hitherto has been a small amount of not more than 1 kg/hr. Moreover, with the process of Patent Document 14 (Japanese Patent Application Laid-Open No. 2003-300936), a column top component (a mixture of methanol and dimethyl carbonate) from a first step reactive distillation column is fed into a second step distillation column, and extractive distillation is carried out using ethylene carbonate. After a mixture of ethylene carbonate and dimethyl carbonate has been obtained as a column bottom component from the second step distillation column, this mixture is then further fed into a third step distillation column, and separation by distillation is carried out so as to obtain dimethyl carbonate as a column top component and ethylene carbonate as a column bottom component from the third step distillation column. That is, with the process of Patent Document 14 (Japanese Patent Application Laid-Open No. 2003-300936), two columns must be used to separate the dimethyl carbonate out from the mixture of methanol and dimethyl carbonate, and hence the equipment is expensive. Furthermore, with this process, four distillation columns must be operated together with one another, and hence it is expected that prolonged stable operation would be difficult.